1. Field of the Invention
The present invention relates to a zoom lens and an optical device using the zoom lens, which is particularly suitable for use in optical equipment such as electronic cameras, including a video camera and a digital still camera, and film cameras. Also, the present invention relates to a zoom lens, which has a short overall length and a large aperture ratio with the F-number being about 1.6 at the wide-angle end while it has a zoom magnification ratio as high as on the order of 10, and an optical device using the zoom lens.
2. Description of the Related Art
Hitherto, a zoom lens having a relatively small overall lens system and a relatively high zoom magnification ratio has been proposed in, e.g., Japanese Patent Laid-Open Nos. 56-114920 and 58-160913 (U.S. Pat. No. 4,720,180). This zoom lens comprises four lens units, i.e., a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power, which are arranged in this order from the object side. The first, second and fourth lens units are moved for zooming. With such an arrangement, a relatively high zoom magnification ratio is provided while the overall lens system is of a relatively small size.
Japanese Patent Laid-Open Nos. 62-24213 (U.S. Pat. No. 4,859,042) and 62-247316 propose a zoom lens similarly comprising four lens units, i.e., a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power, which are arranged in this order from the object side. In this zoom lens, however, the second lens unit is moved for zooming and the fourth lens unit is moved to compensate for a shift of an image plane (field shift) upon zooming. Further, the fourth lens unit also serves for focusing.
Also, Japanese Patent Laid-Open No. 4-14007 (U.S. Pat. No. 5,134,524) discloses a zoom lens comprising a first lens unit having positive refractive power, a second lens unit having negative refractive power, an aperture stop, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power, which are arranged in this order from the object side. For zooming from the wide-angle end to the telephoto end, the first lens unit is moved toward the object side and the second lens unit is moved toward the image (plane) side. The fourth lens unit is moved for zooming and focusing. Further, the aperture stop is moved toward the image side for zooming from the medium focal length to the telephoto end.
Japanese Patent Laid-open Nos. 58-129404 and 61-258217 (U.S. Pat. No. 4,776,680) disclose a zoom lens comprising five lens units, i.e., a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, a fourth lens unit having positive refractive power, and a fifth lens unit having negative refractive power, which are arranged in this order from the object side. The fifth lens unit or a plurality of lens units including the fifth lens unit is moved for focusing.
Japanese Patent Laid-Open No. 4-301811 discloses a zoom lens comprising five lens units, i.e., a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, a fourth lens unit having positive refractive power, and a fifth lens unit having negative refractive power, which are arranged in this order from the object side, with an aperture stop situated between the second and third lens units. For zooming from the wide-angle angle end to the telephoto end, the first lens unit is moved toward the object side, the second lens unit is moved toward the image side, and the aperture stop is kept fixed. The fourth lens unit is moved not only to compensate for shift of an image plane upon zooming, but also to serve for focusing.
U.S. Reissue Pat. No. 32,923 discloses a zoom lens comprising a first positive lens unit, a second negative lens unit, an aperture stop, a third positive lens unit, and a fourth positive lens unit, which are arranged in this order from the object side. For zooming, the first and fourth lens units are moved in the same direction while the stop is kept fixed.
Japanese Patent Laid-Open Nos. 4-14006 (U.S. Pat. No. 5,134,524) and 4-358108 (U.S. Pat. No. 5,341,243) disclose a zoom lens comprising a first lens unit having positive refractive power, a second lens unit having negative refractive power, an aperture stop, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power, which are arranged in this order from the object side. For zooming from the wide-angle end to the telephoto end, the first lens unit is moved toward the object side and the second lens unit is moved toward the image side. The fourth lens unit is moved for zooming and focusing, while the aperture stop is always kept fixed during the zooming.
Japanese Patent Laid-Open No. 11-242160 discloses a zoom lens comprising a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power, which are arranged in this order from the object side. For zooming from the wide-angle end to the telephoto end, the first and fourth lens units are moved toward the object side and the second lens unit is moved toward the image side. The third lens unit and an aperture stop are always kept fixed during the zooming.
Moreover, Japanese Patent Laid-Open No. 5-143178 discloses a zoom lens comprising a first lens unit having positive refractive power, which is kept fixed during zooming, a second lens unit having negative refractive power, an aperture stop, a third lens unit having positive refractive power, and a fourth lens unit constituted by a single lens having positive refractive power, which are arranged in this order from the object side. The aperture stop is moved for zooming.
Recently, there has been a demand for a zoom lens used in optical equipment, such as a single-lens reflex camera and a video camera, which has a zoom magnification ratio as high as on the order of 10 and also has a small overall lens system.
In general, to obtain a zoom lens having a high zoom magnification ratio, a short overall lens system and high optical performance over a full zooming range, a lens arrangement, a refractive power of each lens unit, etc. must be properly set.
Unless, for example, moving conditions of each lens unit in zooming, a refractive power of each lens unit, a lens arrangement of each zooming lens unit, a selection of one or more lens units for focusing and a lens arrangement of each selected lens unit, an optical action of an aperture stop, etc. are properly set, aberrations that occur upon zooming and focusing increase to such an extent that it is difficult to obtain a high-quality image while realizing a high zoom magnification ratio.
In a zoom lens, it is a generally known principle that intensifying refractive power of each lens unit is effective to increase a zoom magnification ratio and shorten the overall lens system, because the amount by which each lens unit must be moved for obtaining a predetermined zoom magnification ratio is reduced correspondingly.
Simply intensifying refractive power of each lens unit, however, raises a problem that variations of aberrations upon zooming are increased and it is difficult to obtain high optical performance over a full zooming range.
Meanwhile, by employing rear focusing in a zoom lens, advantageous features result, for example, in that the size of an overall lens system is reduced, more rapid focusing is realized, and macro (close-up) shooting is facilitated.
However, variations of aberrations upon zooming are increased, which makes it very difficult to obtain high optical performance over the entire range of a distance to an object, ranging from an infinite object to a proximity object, while realizing a size reduction of the overall lens system.
Accordingly, it is an object of the present invention to provide a small-sized zoom lens and an optical device using the zoom lens. By properly setting moving conditions of predetermined lens units in zooming, a refractive power and a lens arrangement of each lens unit, etc., a zoom lens having a short overall lens system and having high optical performance over a full zooming range with a simplified lens arrangement while having a high zoom magnification ratio is obtained.
Another object of the present invention is to provide a zoom lens in which the size of the overall lens system is reduced in spite of a zoom magnification ratio being as high as on the order of 10. Still another object of the present invention is to provide a zoom lens which has high optical performance and comprises fewer lenses while having a large aperture ratio, with the F-number being about 1.6.
To achieve the above objects, according to a first aspect of the present invention, a zoom lens comprises a first lens unit having positive refractive power; a second lens unit having negative refractive power; an aperture stop; a third lens unit having positive refractive power and kept fixed during zooming; a fourth lens unit having positive refractive power; and a fifth lens unit having negative refractive power, which are arranged in this order from the object side. In zooming from the wide-angle end to the telephoto end, the second lens unit is moved toward the image plane side such that a spacing between the first lens unit and the second lens unit is increased, and the aperture stop is also moved.
Preferably, the first lens unit is moved toward the object side in the zooming from the wide-angle end to the telephoto end.
Preferably, the fourth lens unit is moved in compensating for shift of an image plane upon zooming and in focusing.
Preferably, the fourth lens unit is moved to follow a locus convex toward the object side in zooming.
Preferably, the aperture stop is moved toward the image plane side in the zooming from the wide-angle end to the telephoto end.
Preferably, in the zooming from the wide-angle end to the telephoto end, the aperture stop is moved toward the image plane side in the range from a medium position of a zooming range to the telephoto end.
Preferably, in the zooming from the wide-angle end to the telephoto end, the aperture stop is moved to follow a locus that is located nearest to the object side at a medium position of a zooming range.
Preferably, assuming that distances between the second lens unit and the aperture stop at a zoom position at which the aperture stop is positioned farthest from the third lens unit along an optical axis and the telephoto end are respectively L2s, L2t, and distances between the aperture stop and the third lens unit at the zoom position and the telephoto end are respectively Ls3, L3t, the following conditional formula is satisfied:
0.4 less than (L2sxe2x88x92L2t)/(Ls3xe2x88x92L3t) less than 1.3
Preferably, assuming that the focal lengths of an overall lens system at the wide-angle end and the telephoto end are respectively fw, ft, the focal length of the second lens unit is f2, and fA is defined as given below;
fA={square root over (fwxc2x7ft)}
the following conditional formula is satisfied:
0.19 less than |f2/fA| less than 0.30
Preferably, assuming that the distance from a first lens surface on the object side to a paraxial image plane in a state of focusing on an infinite object at the wide-angle end is L on condition that the distance from a final lens surface to the image plane is calculated in terms of air, the focal lengths of an overall lens system at the wide-angle end and the telephoto end are respectively fw, ft, and fA is defined as given below;
fA={square root over (fwxc2x7ft)}
the following conditional formula is satisfied:
2.22 less than L/fA less than 3.36
Preferably, assuming the amounts by which the first lens unit and the second lens unit are moved in the zooming from the wide-angle end to the telephoto end are respectively S1, S2, the following conditional formula is satisfied:
0.6 less than S1/S2 less than 1.2
Preferably, the fifth lens unit is fixed during zooming.
Preferably, the third lens unit is moved to have a motion component in a direction perpendicular to an optical axis.
Preferably, the second lens unit comprises a meniscus-shaped negative lens having a concave surface faced toward the image plane side, a negative lens having both concave lens surfaces, and a cemented lens, which are arranged in this order from the object side, the cemented lens being made of a positive lens, which has a more convex surface faced toward the object side than that faced toward the image plane side, and a negative lens, which are cemented to each other.
Preferably, the second lens unit has at least one aspherical surface.
Preferably, assuming that the average Abbe""s numbers of materials of a positive lens and a negative lens in the second lens unit are respectively xcexdp, xcexdn, at least one of the following conditional formulae is satisfied for the average Abbe""s number:
20 less than xcexdp less than 35
36 less than xcexdn less than 65
Preferably, assuming that the average value of refractive index of material of a negative lens in the second lens unit is Nn, the following conditional formula is satisfied:
1.80 less than Nn less than 1.96
Preferably, assuming that the radius of curvature of an i-th lens surface in the second lens unit counted from the object side is RR2i, the following conditional formula is satisfied:
0.60 less than |R22/R23| less than 0.82
To achieve the above objects, according to a second aspect of the present invention, a zoom lens comprises a first lens unit having positive refractive power; a second lens unit having negative refractive power; an aperture stop; a third lens unit having positive refractive power and not moving for zooming; and a fourth lens unit having positive refractive power and including at least one negative lens. In zooming from the wide-angle end to the telephoto end, the second lens unit is moved toward the image plane side such that a spacing between the first lens unit and the second lens unit is increased, and the aperture stop is also moved.
Preferably, assuming that the shortest exit pupil position of an overall lens system in the zooming from the wide-angle end to the telephoto end and in focusing from an infinite object to a proximity object is EP, the focal length of the overall lens system at the wide-angle end is fw, the effective size of an image pickup device is LD, and the half angle xcfx89w of view is expressed by;       ω    ⁢          xe2x80x83        ⁢    w    =            tan              -        1              ⁢          LD              2        ⁢        fw            
the following conditional formula is satisfied:
0 less than |(fwxc2x7tan xcfx89w)/EP| less than 0.13
Preferably, the first lens unit is moved toward the object side in the zooming from the wide-angle end to the telephoto end.
Preferably, the fourth lens unit is moved in compensating for shift of an image plane upon zooming and in focusing.
Preferably, the fourth lens unit is moved to follow a locus convex toward the object side in zooming.
Preferably, in the zooming from the wide-angle end to the telephoto end, the aperture stop is moved toward the image plane side in the range from a medium position of a zooming range to the telephoto end.
Preferably, in the zooming from the wide-angle end to the telephoto end, the aperture stop is moved to follow a locus that is located nearest to the object side at a medium position of a zooming range.
Preferably, in the zooming from the wide-angle end to the telephoto end, the aperture stop is moved to follow the same locus as that of the first lens unit in the range from the wide-angle end to a medium position of a zooming range, and to follow the same locus as that of the second lens unit in the range from the medium position of the zooming range to the telephoto end.
Preferably, assuming that the focal lengths of an overall lens system at the wide-angle end and the telephoto end are respectively fw, ft, the focal length of the second lens unit is f2, and fA is defined as given below;
fA={square root over (fwxc2x7ft)}
the following conditional formula is satisfied:
0.1 less than |f2/fA| less than 0.29
Preferably, assuming that the focal length of an overall lens system at the wide-angle end is fw and the spacing between the principal points of the third lens unit and the fourth lens unit that results in focusing on an infinite object at the telephoto end is e3T, the following conditional formula is satisfied:
1.79 less than e3T/fw less than 2.76
Preferably, assuming that the focal length of an i-th lens unit is fi, the following conditional formula is satisfied:
0.63 less than f3/f4 less than 1.02
Preferably, assuming that distances between the second lens unit and the aperture stop at a zoom position at which the aperture stop is positioned farthest from the third lens unit along an optical axis and the telephoto end are respectively L2s, L2t, and distances between the aperture stop and the third lens unit at the zoom position and the telephoto end are respectively Ls3, L3t, the following conditional formula is satisfied:
0.2 less than (L2sxe2x88x92L2t)/(Ls3xe2x88x92L3t) less than 0.9
Preferably, assuming the amounts by which the first lens unit and the second lens unit are moved in the zooming from the wide-angle end to the telephoto end are respectively M1, M2, the following conditional formula is satisfied:
0.5 less than M1/M2 less than 1.2
Preferably, the second lens unit comprises a meniscus-shaped negative lens having a concave surface faced toward the image plane side, and a cemented lens made of a negative lens having both concave lens surfaces and a positive lens having a convex surface faced toward the object side, which are arranged in this order from the object side.
Preferably, the second lens unit has at least one aspherical surface.
Preferably, assuming that for an i-th lens surface in the second lens unit counted from the object side, R2i represents the radius of curvature when the lens surface is a spherical surface and the radius of paraxial curvature when the lens surface is an aspherical surface, the following conditional formula is satisfied:
0.52 less than |R22/R23| less than 0.92
Preferably, the third lens unit comprises a positive lens and a negative lens having a powerful concave surface faced toward the image plane side, which are arranged in order from the object side.
Preferably, the third lens unit or a part of the lenses of the third lens unit is moved to have a motion component in a direction perpendicular to an optical axis.
To achieve the above objects, according to a third aspect of the present invention, an optical device comprises a zoom lens comprising a first lens unit having positive refractive power, a second lens unit having negative refractive power, an aperture stop, a third lens unit having positive refractive power and kept fixed during zooming, a fourth lens unit having positive refractive power, and a fifth lens unit having negative refractive power, which are arranged in this order from the object side, the second lens unit being moved toward the image plane side such that a spacing between the first lens unit and the second lens unit is increased, and the aperture stop also being moved in zooming from the wide-angle end to the telephoto end; and a housing for supporting the zoom lens.
To achieve the above objects, according to a fourth aspect of the present invention, an optical device comprises a zoom lens comprising a first lens unit having positive refractive power, a second lens unit having negative refractive power, an aperture stop, a third lens unit having positive refractive power and not moving for zooming, and a fourth lens unit having positive refractive power and including at least one negative lens, the second lens unit being moved toward the image plane side such that a spacing between the first lens unit and the second lens unit is increased, and the aperture stop also being moved in zooming from the wide-angle end to the telephoto end; and a housing for supporting the zoom lens.
Preferably, assuming that the shortest exit pupil position of an overall lens system in the zooming from the wide-angle end to the telephoto end and in focusing from an infinite object to a proximity object is EP, the focal length of the overall lens system at the wide-angle end is fw, the effective size of an image pickup device is LD, and the half angle xcfx89w of view is expressed by;       ω    ⁢          xe2x80x83        ⁢    w    =            tan              -        1              ⁢          LD              2        ⁢        fw            
the following conditional formula is satisfied:
0 less than |(fwxc2x7tan xcfx89w)/EP| less than 0.13
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.